Photo-Bleaching Process:
A material is opaque to light of certain wavelength because certain mechanism in this material makes it absorbs photons of that particular wavelength. The absorption sometime induces the degradation or saturation of the light absorption mechanism. This therefore renders the material transparent to the certain wavelength. This process is called photo-bleaching. Most organic dyes photo-bleach. For example, the color of hair fades under prolonged exposure to sunshine.
For many applications, the photo-bleaching should be reversible, i.e., the materials recover their original optical property after the light is turned off. The relaxation process can happen automatically. It can also be triggered by external conditions such as electrical or magnetic field, light at different wavelength, heat, etc.
The photo-bleaching process has a wide range of applications. One non-limiting example is the contrast enhancement material (CEM) in photolithography. The transparency of a CEM varies directly with the intensity of the incident light. In other words, its ability to absorb photons decreases as incident light promotes electrons in the CEM from the ground state into the excited state. A CEM increases the contrast of the image, resulting in improved resolution and depth of focus and reduced interference. These factors in turn allow the fabrication of denser integrated circuits without additional capital equipment investment.
FIG. 1 illustrates a prior art contrast enhancing process in photolithography. The distribution of light intensity from the laser light usually follows Airy pattern. The width of the central peak of the Airy pattern is 1.22λ/NA, where λ is the wavelength of the incident light and NA is the numerical aperture of the optical system. In this illustration, only at the center of the spot, where the incident light is most intense, can the light bleaches through the contrast enhancement layer (CEL). Other parts of the Airy Pattern, including the side-lobes, are filtered out by the CEL. Light catalyzes the photoresist only in the region where it bleaches through. Therefore the resulting line width can be much smaller than the incident wavelength. It is also possible to produce a light pattern with a width smaller than that of the Airy pattern at the cost of putting more power in the side lobes. This is known as apodization. (See co-pending commonly-assigned patent application Ser. No. 10/283,322 entitled “Advanced Exposure Techniques For Programmable Lithography). Apodization can further increase the resolution with the help of CEM. A review of strategies for sub-diffraction limit lithography can be found in an article by S. R. J. Brueck, “International Trends in Applied Optics”, SPIE Press, 2002, pp. 85-109.
A prior art example is CEM for 365 nm photolithography manufactured by Shin-Etsu MicroSi. Conventional CEMs, which are based on organic polymers, however, bleach permanently. They do not recover their original property after the incident light is turned off. For lithography processes such as multi-exposure lithography and programmable lithography, reversible contrast enhancement materials (R-CEM) are more relevant. With these lithography methods and a reversible contrast enhancement layer, it is possible to print features finer and denser than the conventional diffraction limit can. See U.S. Pat. No. 6,291,110 B1, entitled “Methods for Transferring a Two-dimensional Programmable Exposure Pattern for Photolithography”; and commonly-assigned provisional patent application No. 60/463,626, entitled “Methods of Improving Resolution in a Photolithographic System Using Multi-image Transfer Through Layers to a Photo-resist”;
We propose a new type of photo-bleachable and reversible photo-bleachable materials based on nano-particles of semiconductors. Such type of materials provides reversibility, i.e., they can recover their original properties after the incident light is turned off. Moreover, these materials cover a broad spectrum of the wavelength from far infrared to deep ultra-violet, including the entire spectrum of optical applications.
Semiconductor Nano-Particles:
Nano-particles are loosely defined as powders with diameter ranging from 1 nm to 100 nm. Since they have only been the focus of research in the last two decades, they assumes different names, such as quantum dot, quantum sphere, quantum crystallite, nano-crystal, micro-crystal, colloidal particle, nano-particle, nano-cluster, Q-particle or artificial atom. They also assume different shapes, spherical, cubical, rod-like, tetragonal, single or multiple-walled nano-tubes, etc.
Due to their small size, nano-particles often possess dramatically different physical properties from their bulk counterparts. Nano-particles have a wide range of applications, from metallurgy, chemical sensor, pharmaceutical, painting industry to cosmetics industry. Thanks to the rapid development in synthesis methods in the last two decades, they have now entered into microelectronic and optical applications. Nano-particles of a variety of semiconductors, including those most common ones have been successfully synthesized. A non-exhaustive list includes: C, Si, Ge, CuCl, CuBr, CuI, AgCl, AgBr, AgI, Ag2S, CaO, MgO, ZnO, MgxZn1-xO, ZnS, HgS, ZnSe, CdS, CdSe, CdTe, HgTe, PbS, BN, AlN, GaN, AlxGa1-xN, GaP GaAs, GaSb, InP, InAs, InxGa1-xAs, SiC, Si1-xGex, Si3N4, ZrN, CaF2, YF3, Al2O3, SiO2, TiO2, Cu2O, Zr2O3, ZrO2, SnO2, YSi2, GaInP2, Cd3P2, Fe2S, Cu2S, CuIn2S2, MoS2, In2S3, Bi2S3, CuIn2Se2, In2Se3, HgI2, PbI2 and their various isomers and alloys. They have revealed very interesting electrical and optical properties.
In a semiconductor material, the possible energy states for electrons are grouped into energy bands. These energy bands are separated by band-gaps where no electron states are allowed. The highest populated energy band is called the valence band and the lowest unpopulated energy band is called the conduction band. When a photon with energy less than the band-gap that separates the valence band and conduction band is incident on the semiconductor material it will not be absorbed. However, if the photon has energy higher than the band-gap, it will be absorbed by promoting an electron from the top of the valence band to the bottom of the conduction band. Meanwhile this process will leave an empty electron state, a hole, at the top of the valence band. In reality, the electron-hole pair created by the photon forms an entity resembling a hydrogen atom, called an “exciton”. The Coulomb attraction between the electron and hole will lower their energy. Therefore photons with energy slightly less than the band-gap can be absorbed. The lowest energy where absorption occurs is called the absorption edge. If all the electron and hole pairs corresponding to certain photon energy are excited to form excitons, no more absorption can occur, and the material is “bleached”. Hence semiconductors are bleachable materials.
Excited carriers in a semiconductor have a finite life-time. After this time the semiconductor relaxes by recombination of electrons and holes. Hence after the light is turned off, a bleached semiconductor can recover its original optical property. In other word, the bleaching process in a semiconductor is reversible. The relaxation process can also be triggered by external conditions such as electrical field, magnetic field, light with different wavelength, heat, etc.
The amount of light with a given energy that a semiconductor can absorb is proportional to its density of states (DOS), that is, the number of electron and hole states available at this energy. Semiconductors in bulk and thin film form do not bleach easily. The bulk DOS is large and requires a high light intensity, it is shown in FIG. 2a. One way to decrease the density of state is through the quantum size effects. When the size of a semiconductor becomes very small, the energy levels of electrons and holes are no longer continuous. They are quantized into discreet levels. Usually when the thickness of a thin film approaches several tens of nanometer, the material becomes “two dimensional” and its DOS become stair-like, as seen in FIG. 2a. FIG. 2b depicts the DOS of a “one dimensional” wire with a thickness less than a few tens of nanometer. Finally, in FIG. 2c when the material reaches the “zero dimensional” quantum-dot, its density of state becomes discreet delta functions, similar to atomic or molecular energy levels. Although in reality the states of a nano-particle are a little smeared out and look like the dotted lines in FIG. 2d, the DOS usually is still significantly smaller than the bulk.
Furthermore, nano-particles can be easily dispersed and diluted hence it is much easier to observe photo-bleaching. Strong power dependent absorption has been observed in CdSxSe1-x nano-particles. As described in the book entitled “Optical Properties of Semiconductor Nanocrystals” to S. V. Gaponenko, Cambridge Univ. Press, 1998, Chapter 6.
Another distinctive feature of semiconductor nano-particle is the tunabillity of the absorption edge by its size. In a nano-particle, the electrons and holes are much closer to each other in this confined space than in bulk. Therefore the Coulomb interaction between electrons and holes are much stronger than in the bulk. For optical applications, it is convenient to categorize the semiconductor nano-particles relative to their bulk exciton size aB. If the size of a particle a>aB, it is in the weak confinement regime. If a<aB, then it is in the strong confinement regime.
In the weak confinement regime, the nano-particles can still be treated as bulk materials. The quantization of the electron and hole energy is much less than the quantization of the exciton energy levels. Therefore the change in optical properties is mainly due to the change of exciton energy. A qualitative formula for the exciton ground state is expressed in equation (1), as described in an article by Al. L Efros, A. L. Efros, Sov. Phys. Semicon. 1982,16:772-78.
                    ℏω        =                              E            g                    -                      E            x                    +                                                    ℏ                2                            ⁢                              π                2                                                    2              ⁢                              (                                                      m                    e                                    +                                      m                    h                                                  )                            ⁢                              a                2                                                                        (        1        )            where Eg is the bulk value for band-gap, Ex is the bulk value for exciton ground energy, me and mh are the effective masses of electron and hole respectively, h is the Plank constant and ω is the angular frequency of the photon. It can be immediately seen from this equation that the absorption peak energy corresponding to the exciton increases rapidly with size reduction.
In the strong confinement case, the nano-particle can not be treated as bulk materials and the Coulomb interaction can not be described as a hydrogen-like entity. The energy levels for electrons and holes no longer form continuous bands. But rather, they form well separated discreet levels, like in atoms or molecules. A largely simplified model for absorption spectrum of nano-particles in the strong confinement regime is described in equation (2), as described in an article by L. E. Brus, J. Chem. Phys. 1983, 79:5566-71.
                    ℏω        =                              E            g                    -                                    1.8              ⁢                              q                2                                                    κ              ⁢                                                          ⁢              a                                +                                                    ℏ                2                            ⁢                              π                2                                                    2              ⁢                              (                                                      m                    e                                    +                                      m                    h                                                  )                            ⁢                              a                2                                                                        (        2        )            where Eg is the bulk band-gap, me and mh are the effective masses of the electron and hole respectively, q is the electron charge, κ is a constant, h is the Plank constant and ω is the angular frequency of the photon. In equation (2) the absorption edge again increases rapidly with decreasing size. This effective band-gap widening effects has been observed in many materials. As an extreme example, Cd3P2, its band-gap increases from its bulk room temperature value of 0.5 eV to about 2 eV for nano-particles with 2.7 nm diameter.
There is a variety of ways of manufacturing nano-particles. An non-exhaustive list includes chemical vapor deposition (CVD), chemical mechanical polishing (CVP), self-organized growth on vicinal substrates in various film deposition techniques, laser ablation, plasma assisted decomposition, sol-gel synthesis, electro-explosion, and chemical synthesis. Nano-particles with average particle size as small as <1 nm with different shapes can be produced. Nano-particles with core in the middle and shell made of different materials can also be produced. If the standard deviation of the size distribution of nano-particles is smaller than 5%, it is called a mono-dispersion. It is also feasible to manipulate the mono-disperse nano-particles into ordered or disordered close-packed assemblies possessing very different properties as their bulk properties. A review of mono-disperse nano-particles can be found in an article “Synthesis and Characterization of Monodisperse Nanocrystals and Close-Packed Nanocyrstal Assemblies”, C. B. Murray, C. R. Kagan and M. G. Bawendi, Annu. Rev. Mater. Sci. 2000, 30:545-610.
When electrons from inner shells of semiconductor atoms are excited into the conduction gap, they can absorb lights in the EUV and soft X-ray spectra. The absorption of inner shell electrons bleaches in the same manner as those valence band electrons. Therefore reversible photo-bleachable materials for EUV or soft X-ray can also be developed based on semiconductor nano-particles with proper energy levels.
In summary, advantages of using semiconductor nano-particles as reversible photo-bleachable materials include:                Most semiconductors can be fabricated into nano-particles with relatively low cost. The different band-gap of different semiconductors can cover from far infrared to ultra-violet. For example, GaAs can be used for infrared light, AlGaAs and InGaN for the visible light, and AlGaN and MgZnO for ultra-violet light. Even for application like 193 nm and 157 nm UV photolithography, suitable material systems, such as MgZnO, exist.        Easy to apply. Nano-particles are easy to be formed into thin film or bulk with different shapes. Thus reduces the overall cost of optical applications.        The absorption edge of nano-particles can be tuned by changing their size. This adds another degree of freedom and simplicity to the design. For example, in the application of UV photolithography, same CEM can work at 365 nm and 248 nm, even 193 nm, only with different particle size.        Electronic states in nano-particles are highly localized within the particles. Therefore an assembly of nano-particles resolve light distribution with high spatial frequency.        The relaxation time can be manipulated from pico-second to seconds, provided that certain measures are taken. It covers time scale required for most optical applications. An example is shown in Gribkovskii et al, “Optical Nonlinearity of Semiconductor Microcrystal CdSxSe1-x Under the Action of Picosecond and Nanosecond Laser Pulses”, Phys. Stat. Sol. (b) 158: 359-66 (1988), where the surface passivated material relaxed in less than a nano-second. An example on the other extreme can be seen in “Organic-Capped ZnO Nanocrystals”, M. Shim and Philippe Guyot-Sionnest, J. Am. Chem. Soc., 2001, 123, 11651. In this article it is demonstrated that in ZnO nanocrystal relaxation time varies from minutes to days.        